Para-xylene is an important starting material for manufacturing terephthalic acid, which is itself a valuable intermediate in the production of synthetic polyester fibers, films, and resins. These polyester materials have many practical, well known uses, such as in fabrics, carpets, and apparel.
One known route for the manufacture of para-xylene is by the methylation of benzene and/or toluene. For example, U.S. Pat. No. 6,504,072 discloses a process for the selective production of para-xylene which comprises reacting toluene with methanol under alkylation conditions in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa) wherein said porous crystalline material has undergone prior treatment with steam at a temperature of at least 950° C. to adjust the Diffusion Parameter of said material to about 0.1-15 sec−1. The reaction can be carried out in a fixed, moving, or fluid catalyst bed.
Although the reaction of toluene and methanol, particularly using the highly steamed catalyst described in the '072 patent, is selective to the production of para-xylene, in commercial applications it is critical to maximize product selectivity and feedstock utilization. Thus, competing reactions include reaction of methanol with itself to produce light paraffins and olefins and coke, which reactions involve loss of valuable methanol feed. There is therefore considerable interest in developing methods to further enhance the selectivity of the benzene/toluene methylation process.
For example, U.S. Pat. No 6,642,426 discloses production of xylene by fluidized bed alkylation of toluene with methanol, in which improved conversion and selectivity is realized when the methanol is injected in stages into the fluidized bed at one or more locations downstream from the location of aromatic reactant introduction into the fluidized bed. In addition, the '246 patent teaches that methanol utilization is enhanced by using a molar excess of the aromatic feed as compared to the methanol feed.
Another important parameter in the reaction of benzene and/or toluene with methanol to produce para-xylene is temperature, with relatively high temperatures, typically between 450° C. and 700° C., being required to maximize conversion. As a result, the aromatic and methanol feeds are preheated before being supplied to the alkylation reactor(s), with the exothermic heat generated by the alkylation reaction generally being sufficient to maintain the reaction temperature at the desired value. In practice, however, there are limits on the temperatures to which the different feeds can be preheated. For example, in the case of the benzene/toluene feed, the preheating temperature is limited by the coking rates in the preheater which, depending on factors such as heat flux, stream composition and heat transfer surface metallurgy, will generally be about 550° C. In the case of the methanol feed, decomposition to carbon oxides, hydrogen and methane will generally limit the preheating temperature to about 220° C.
In a conventional reactor system, the feed rates of the methanol and aromatic feeds and the molar ratio of aromatic to methanol are set at the desired values and the reaction temperature is controlled by adjusting the amount of heat supplied to feeds in the preheaters. However, this approach suffers from a problem in that, for a given molar ratio of aromatic to methanol, it may not be possible to achieve the optimal reaction temperature before the maximum feed preheating constraints are reached. This will result in the methylation reaction being operated at a temperature less than that required for maximum selectivity to the desired para-xylene product.
According to the invention, this problem is addressed by maintaining the preheating temperatures of the methanol and aromatic feeds at or near their desired maximum values and then adjusting the molar ratio of the methanol to aromatic feeds to achieve the desired reaction temperature. For example, since the conversion of methanol in the process, whether by alkylation or production of light gases, is exothermic, an increase in the methanol to aromatic molar ratio can be used to increase the supply of heat to the reaction and hence raise the reaction temperature. Alternatively, since conversion of methanol is the rate limiting step, a decrease in the methanol to aromatic molar ratio can be used to decrease the supply of heat to the reaction and hence lower the reaction temperature. Controlling the reaction temperature in this manner guarantees that, for a given desired reaction temperature and maximum value of the feed preheating temperatures, the lowest possible methanol to aromatic molar ratio will be employed. This maintains the methanol concentration in the reactor at its lowest possible value, resulting in the highest possible selectivity to the desired xylene product.